Plastics are a climate problem. Making precursors for common plastics, such as ethylene and carbon monoxide (CO), consumes fossil fuels and releases plenty of carbon dioxide (CO2). In recent years, chemists have devised bench-top reactors called electrochemical cells that aim to reverse the process, starting with water and waste CO2 from industrial processes and using renewable electricity to turn them into feedstocks for plastics. But that green vision has a practical problem: The cells often consume highly alkaline additives that themselves take energy to make.